Dual core support problem ?

You can't compare 2 different programs on how they handle dual core. It's already been said, in Dolphin, 1 core is used for the video processing (in most case, the 2nd one) while everything else is on the 1st one (core, DSP,etc) PCSX2 doesn't do that the same way.

Since in most scenario the core is already running at it's maximum capacity, the video doesn't needs to render as much so that's why it kind of idle there, waiting to be called to render something.